This invention relates to apparatus for piercing slits in plastic films and more particularly to an apparatus and blade for piercing a series of slits across the width of a tube of thermo-plastic film material for the manufacture of rolls of continuous but separable plastic bags.
In the manufacture of bottom sealed thermo-plastic bags, it is necessary to process a tube of material through an apparatus which produces a heat seal across the width of the tube and an adjacent series of slits. The heat seal provides the bottom closure for one bag and the series of slits form a parting line for separating the one bag from the other and forming a top opening for the next bag when the bags are separated by tearing along the line of the slits. As is well known in the industry, the perforated tubular web or film, as it is commonly referred to, must have sufficient strength in the remaining areas between the sheets to maintain integrity of the roll until it is desired to separate the bags.
A common technique for heat sealing and perforating a tubular film to form bags is to index the film as it emerges from the blown film extrusion through an in-line bag machine such as commercially available from the Gloucester Universal Bag Machine, marketed by Gloucester Engineering Company, located in Gloucester, Mass. In a machine of this type, a reciprocating serrated blade and sealing element are reciprocated into and out of contact with the tubular film to heat seal the film to form the bottom of one bag while piercing a series of slits across the width of the film adjacent the heat sealed bottom to form the top opening of the next bag when separated from the tubular film. This in-line bag machine functions satisfactorily for many types of films such as low density polyethylene (LDPe) film. However, it has been found that machines of this type are not entirely satisfactory for linear low density polyethylene (LLDPe) in that the blade must be carefully maintained to provide satisfactory penetration, and even the slightest dulling tends to push the film downwardly into the slot underlying the film, resulting in partial perforations, or if the film is fully supported in the underlying slot, it tends to weld together the edges of the slit formed by the blade penetration. Furthermore, the downward pushing action, particularly of a dull blade, increases the film tension in the heat sealing region and adversely effects the quality of the closure attained.
In U.S. Pat. No. 5,001,956, issued Mar. 26, 1991, I have disclosed a blade for use in a piercing or slitting operation which provides an improved slitting action to form arcuate slits across the width of the tubular thermo-plastic film which eases the piercing force required, and which prevents the sealing together of the edges of the slits across the tubular web so that when the bag is separated from the continuous web, the top can be readily opened.